Method and system for coexistence of radar and communication systems

ABSTRACT

A method for scheduling wireless transmissions in a wireless communications network in the presence of pulsed interference includes receiving pulsed interference information, determining transmission opportunities based on the pulsed interference information, and when a transmission time interval (TTI) is less than a time of an opportunity of the transmission opportunities, transmitting a wireless signal during the opportunity.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present invention claims priority to U.S. Provisional ApplicationNo. 61/931,849, filed Jan. 27, 2014, which is incorporated by referenceherein for all purposes.

This invention was made with government support under contract numberHR0011-13-C-0082, awarded by the Defense Advanced Research ProjectsAgency (DARPA). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The explosive growth of mobile communications has made spectrum a scarcecommodity and increased the focus on efficient utilization of thewireless spectrum. In this regard, a number of initiatives have beenlaunched to share under-utilized spectrum assigned to government andmilitary agencies with commercial entities. One of these initiativesaddresses sharing the spectrum originally assigned to radar systems withcommercial communication networks.

Radar systems typically transmit signals with very high power levels andlow duty cycles. A possible strategy for spectrum sharing is to maintainan exclusion zone around the radar systems within which commercialnetworks cannot operate. The exclusion region is determined by thegeographical separation required to prevent mutual interference betweenthe radar and communication systems. However, given the large transmitpower levels of radar signals, in the absence of other interferencemitigation techniques, the size of the exclusion regions limit thebenefits of spectrum sharing.

BRIEF SUMMARY OF THE INVENTION

Embodiments of this disclosure relate to interference mitigationtechniques that reduce the mutual interference between the radar andcommunications systems. Although specific embodiments are described withrespect to radar systems, other embodiments are applicable to coexistwith other types of pulsed transmissions.

In an embodiment, a method for coexistence with a pulsed interferencesource in a communications network includes identifying transmissiontime intervals (TTIs) that are affected by pulses from the pulsedinterference source and changing a transmission parameter for the TTIsthat are affected by pulses. The method may further include transmittingthe TTIs that are affected by pulses using a first transmission schemeand transmitting TTIs other than TTIs that are affected by pulses usinga second transmission scheme that has higher transmission rate than thefirst modulation scheme. The communications network may be a Long TermEvolution communications network, and the transmission scheme may be aModulation and Coding Scheme (MCS).

The method may further include determining a signal to interference plusnoise ratio (SINR) for the TTIs that are affected by pulses, and thetransmission parameter may be changed based on the determined SINR. Inan embodiment, the SINR is calculated based on a location and a transmitpower of the pulsed interference source. In another embodiment, the SINRis determined based on measurements by a receiver of the communicationsnetwork.

In an embodiment, the method includes transmitting the TTIs that areaffected by pulses at a first forward error correction (FEC) rate andtransmitting TTIs other than TTIs that are affected by pulses at asecond FEC rate that is higher than the first FEC rate. Changing thetransmission parameter may include changing a modulation scheme andchanging an FEC rate. The method may further include identifying a dwelltime of consecutive interfering pulses and lowering one of a modulationscheme and an FEC rate during an opportunity between adjacent dwelltimes.

In an embodiment, a method for coexistence with a pulsed interferencesource in a communications network includes receiving a communicationssignal, determining a first portion of the communications signal that isaffected by the pulsed interference source, blanking the first portionof the communications signal, and processing the communications signal.Determining the first portion of the communications signal includesdividing a transmission time interval (TTI) into a plurality ofsubunits. The method may further include determining energy values forthe subunits, determining an average energy for the subunits, andcalculating ratios between the energy value for each subunit and theaverage energy.

In an embodiment, the method further includes comparing the ratios to athreshold value, and when a ratio exceeds the threshold value, blankingthe subunit associated with the ratio that exceeds the threshold value.The method may further include comparing a size of the first portion toa threshold value, wherein blanking the first portion is performed onlywhen the size is greater than the threshold value. Comparing the size ofthe first portion to a threshold value may include comparing a number ofsymbols that are affected by the pulsed interference source to thepredetermined value.

In the coexistence method, determining the first portion of thecommunications signal may include comparing pulse times of the pulsedinterference source to transmission times of communications signals. Thepulse times of the pulsed interference source may be input into thecommunications network, or they may be determined by measuring pulses ata receiver in the communications network.

In an embodiment, a method for scheduling wireless transmissions in awireless communications network in the presence of pulsed interferenceincludes receiving pulsed interference information, determiningtransmission opportunities based on the pulsed interference information,and when a transmission time interval (TTI) is less than a time of anopportunity of the transmission opportunities, transmitting a wirelesssignal during the opportunity. Determining transmission opportunitiesmay include identifying off-times between radar pulses, and theopportunities may be during the off-times.

In an embodiment, determining transmission opportunities includesdetermining a buffer time between pulse times, and the transmissionopportunities are off times between pulses minus the buffer. The buffertime may be a receive window in which a radar receives reflections or asaturation time during which a receiver is saturated by a receivedpulse. Determining transmission opportunities may include determiningradar dwell times.

In an embodiment, determining radar dwell times includes identifying aplurality of consecutive radar pulses and comparing the plurality ofconsecutive radar pulses to a threshold value, and the dwell time is atime during which a portion of the consecutive radar pulses exceeds thethreshold value. Transmission opportunities may be times between dwelltimes.

In an embodiment, determining transmission opportunities includescomparing a TTI to an off time between pulses, when the TTI is less thanthe time between pulses, identifying a transmission opportunity in thetime between pulses, and when the TTI is greater than the time betweenpulses, determining whether a dwell time is present in a pulse pattern.

Receiving pulsed interference information may include measuring channelquality variance at a receiver, when channel quality passes below athreshold value, determining an associated time point as a start of adwell time, and when channel quality passes above the threshold value,determining an associated time point as an end of a dwell time, and theopportunity may be between consecutive dwell times.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication system according to anembodiment.

FIG. 2 illustrates a network resource controller according to anembodiment.

FIG. 3A illustrates a Pulse Repetition Interval of a radar signal, andFIG. 3B illustrates dwell cycles of a radar signal.

FIG. 4 illustrates a communications network experiencing interferencefrom a radar system.

FIG. 5 illustrates an embodiment of a process for characterizing pulsedtransmissions.

FIG. 6 illustrates an embodiment of a process for changing transmissionparameters.

FIG. 7 illustrates wireless transmissions and radar transmissions.

FIG. 8 illustrates an embodiment of a process for blanking signals.

FIG. 9 illustrates another embodiment of a process for blanking signals.

FIG. 10 illustrates an embodiment of an intra-dwell transmission.

FIG. 11 illustrates an embodiment of a process for transmissions in thepresence of radar.

FIG. 12 illustrates an embodiment of an inter-dwell transmission.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of this disclosure can be implemented in numerous ways,including as a process; an apparatus; a system; a composition of matter;a computer program product embodied on a computer readable storagemedium; and/or a processor, such as a processor configured to executeinstructions stored on and/or provided by a memory coupled to theprocessor. In general, the order of the steps of disclosed processes maybe altered within the scope of the invention. Unless stated otherwise, acomponent such as a processor or a memory described as being configuredto perform a task may be implemented as a general component that istemporarily configured to perform the task at a given time or a specificcomponent that is manufactured to perform the task. As used herein, theterm ‘processor’ refers to one or more devices, circuits, and/orprocessing cores configured to process data, such as computer programinstructions.

A detailed description of embodiments is provided below along withaccompanying figures. The scope of this disclosure is limited only bythe claims and encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding. These detailsare provided for the purpose of example and the invention may bepracticed according to the claims without some or all of these specificdetails. For the purpose of clarity, technical material that is known inthe technical fields related to the invention has not been described indetail so that the invention is not unnecessarily obscured.

FIG. 1 illustrates a networked communications system 100 according to anembodiment of this disclosure. As depicted, system 100 includes a datacommunications network 102, one or more base stations 106 a-e, one ormore network resource controller 110 a-c, and one or more User Equipment(UE) 108 a-m. As used herein, the term “base station” refers to awireless communications station provided in a location and serves as ahub of a wireless network. The base stations may include macrocells,microcells, picocells, and femtocells.

In a system 100 according to an embodiment, the data communicationsnetwork 102 may include a backhaul portion that can facilitatedistributed network communications between any of the network controllerdevices 110 a-c and any of the base stations 106 a-e. Any of the networkcontroller devices 110 a-c may be a dedicated Network ResourceController (NRC) that is provided remotely from the base stations orprovided at the base station. Any of the network controller devices 110a-c may be a non-dedicated device that provides NRC functionality amongothers. The one or more UE 108 a-m may include cell phone devices 108a-i, laptop computers 108 j-k, handheld gaming units 108 l, electronicbook devices or tablet PCs 108 m, and any other type of common portablewireless computing device that may be provided with wirelesscommunications service by any of the base stations 106 a-e.

As would be understood by those skilled in the Art, in most digitalcommunications networks, the backhaul portion of a data communicationsnetwork 102 may include intermediate links between a backbone of thenetwork which are generally wire line, and sub networks or base stations106 a-e located at the periphery of the network. For example, cellularuser equipment (e.g., any of UE 108 a-m) communicating with one or morebase stations 106 a-e may constitute a local sub network. The networkconnection between any of the base stations 106 a-e and the rest of theworld may initiate with a link to the backhaul portion of an accessprovider's communications network 102 (e.g., via a point of presence).

In an embodiment, an NRC has presence and functionality that may bedefined by the processes it is capable of carrying out. Accordingly, theconceptual entity that is the NRC may be generally defined by its rolein performing processes associated with embodiments of the presentdisclosure. Therefore, depending on the particular embodiment, the NRCentity may be considered to be either a hardware component, and/or asoftware component that is stored in computer readable media such asvolatile or non-volatile memories of one or more communicating device(s)within the networked communications system 100.

In an embodiment, any of the network controller devices 110 a-c and/orbase stations 106 a-e may function independently or collaboratively toimplement processes associated with various embodiments of the presentdisclosure.

In accordance with a standard GSM network, any of the network controllerdevices 110 a-c (NRC devices or other devices optionally having NRCfunctionality) may be associated with a base station controller (BSC), amobile switching center (MSC), a data scheduler, or any other commonservice provider control device known in the art, such as a radioresource manager (RRM). In accordance with a standard UMTS network, anyof the network controller devices 110 a-c (optionally having NRCfunctionality) may be associated with a NRC, a serving GPRS support node(SGSN), or any other common network controller device known in the art,such as an RRM. In accordance with a standard LTE network, any of thenetwork controller devices 110 a-c (optionally having NRC functionality)may be associated with an eNodeB base station, a mobility managemententity (MME), or any other common network controller device known in theart, such as an RRM.

In an embodiment, any of the network controller devices 110 a-c, thebase stations 106 a-e, as well as any of the UE 108 a-m may beconfigured to run any well-known operating system, including, but notlimited to: Microsoft® Windows®, Mac OS®, Google® Chrome®, Linux®,Unix®, or any mobile operating system, including Symbian®, Palm®,Windows Mobile®, Google® Android®, Mobile Linux®, etc. Any of thenetwork controller devices 110 a-c, or any of the base stations 106 a-emay employ any number of common server, desktop, laptop, and personalcomputing devices.

In an embodiment, any of the UE 108 a-m may be associated with anycombination of common mobile computing devices (e.g., laptop computers,tablet computers, cellular phones, handheld gaming units, electronicbook devices, personal music players, MiFi™ devices, video recorders,etc.), having wireless communications capabilities employing any commonwireless data communications technology, including, but not limited to:GSM, UMTS, 3GPP LTE, LTE Advanced, WiMAX, etc.

In an embodiment, the backhaul portion of the data communicationsnetwork 102 of FIG. 1 may employ any of the following commoncommunications technologies: optical fiber, coaxial cable, twisted paircable, Ethernet cable, and power-line cable, along with any otherwireless communication technology known in the art. In context withvarious embodiments of the invention, it should be understood thatwireless communications coverage associated with various datacommunication technologies (e.g., base stations 106 a-e) typically varybetween different service provider networks based on the type of networkand the system infrastructure deployed within a particular region of anetwork (e.g., differences between GSM, UMTS, LTE, LTE Advanced, andWiMAX based networks and the technologies deployed in each networktype).

FIG. 2 illustrates a block diagram of an NRC 200 that may berepresentative of any of the network controller devices 110 a-c. In anembodiment, one or more of the network controller devices 110 a-c areSON controllers. The NRC 200 includes one or more processor devicesincluding a central processing unit (CPU) 204. The CPU 204 may includean arithmetic logic unit (ALU) (not shown) that performs arithmetic andlogical operations and one or more control units (CUs) (not shown) thatextracts instructions and stored content from memory and then executesand/or processes them, calling on the ALU when necessary during programexecution.

The CPU 204 is responsible for executing computer programs stored onvolatile (RAM) and nonvolatile (ROM) memories 202 and a storage device212 (e.g., HDD or SSD). In some embodiments, storage device 212 maystore program instructions as logic hardware such as an ASIC or FPGA.Storage device 212 may include radar data 214, a parameter adjustmentmodule 216 which performs functions associated with parameteradjustment, and a scheduling module 218 which performs variousscheduling activities. In addition, the storage device 212 may include ablanking module which performs blanking activities, and a processingmodule which processes signals.

The NRC 200 may also include a user interface 206 that allows anadministrator to interact with the NRC's software and hardware resourcesand to display the performance and operation of the networked computingsystem 100. In addition, the NRC 300 may include a network interface 206for communicating with other components in the networked computersystem, and a system bus 310 that facilitates data communicationsbetween the hardware resources of the NRC 200.

In addition to the network controller devices 110 a-c, the NRC 200 maybe used to implement other types of computer devices, such as an antennacontroller, an RF planning engine, a core network element, a databasesystem, or the like. Based on the functionality provided by an NRC, thestorage device of such a computer serves as a repository for softwareand database thereto.

Embodiments of the present disclosure include a system and method formitigating interference from a radar system. Depending upon thescenario, various aspects of the processes described herein may beimplemented individually or in combination with one another.

Radar systems are typically pulsed transmission systems with largetransmit power levels and small duty cycles. FIG. 3A shows an example ofa typical pulsed radar transmission. The time between pulses 302 isreferred to here as the Pulse Repetition Interval (PRI) 304. On-time 306is the duration in the PRI during which the radar pulse is transmitted.Off-time 308 is the remaining duration of the PRI. The Off-time 308between pulses 302 may include a receive window 310. A receive window isa period of time in which a radar system receives reflections from thepulses 302.

Some radar systems monitor their environment for targets by directingtheir transmission and reception in different directions over time. Forexample, in some radar systems, a radar antenna is mounted on a platformthat rotates through a 360 degree arc at a constant rate. Other radarsystems may move along an arc that is less than 360 degrees, but such asystem will still typically have a regular repeating sweep interval.

When a radar transmitter is pointed at a communications network area,interference in the network area will be higher than when transmissionsare pointed away from the network area. This concept is illustrated inFIG. 3B, which shows radar pulses 302 of a moving radar transmitter fromthe perspective of a communications network area. The network areaexperiences peak interference when the radar signal strength is pointedat a receiver in network, and minimum interference when the radar signalstrength is pointed away from the receiver.

FIG. 3B also shows a threshold signal strength value 320, which may alsobe an interference threshold value. A time duration during which signalstrength or interference values exceed the threshold value may bereferred to as a dwell time 322. In some embodiments, wirelesscommunications may not be viable during the dwell time 322, but they maybe viable in the time interval between dwell times 322.

FIG. 4 shows a communication network 400 operating in the same spectrumas a radar system 402. The communication network 400 may have multiplecommunication links formed between transmit and receive nodes. In theembodiment shown in FIG. 4, the communication network 400 is a cellularsystem with base stations 404 and UE devices 406.

The communication network 400 may transmit and receive signals using thesame frequencies used by the radar system 402, or it may transmit andreceive signals in an adjacent frequency band to the frequency band usedby the radar system 402. Even when using adjacent frequency bands, thecommunications system 400 may still experience interference fromout-of-band emissions from the radar system 402. Accordingly,embodiments of this disclosure may be used when a communications system400 communicates in a frequency range which experiences interferencefrom a pulsed interference source.

As explained above with respect to FIG. 3B, depending upon the locationof the communication system 400, the communications system may onlyreceive significant amounts of energy from the radar system 402 duringthose dwell times 322 during which a radar transmitter is pointed in thedirection of the communications system 400. Correspondingly,interference energy may also be received at the radar system 402 fromthe communication system 400, and radar system may experience peakinterference from the communications system when the radar receiver ispointed at the communications network. In other embodiments, thecommunications system 400 may still receive significant amounts ofenergy from the radar system 402 when the radar system is directing itstransmissions in other directions, depending on the antenna pattern ofthe radar system.

In order to coexist with a radar system, it is desirable for acommunications network to be aware of certain aspects of radartransmissions. In an embodiment, known radar parameters such asfrequency, PRI 304, on time 306, off time 308, signal strength, rotationperiod, etc. may be input directly into the communications system 400.However, interference will vary over larger network areas, so parametersthat are sufficient to coexist with radar in one sector of the networkmay not be sufficient for a different sector of the network.

Therefore, in an embodiment, one or more network element such as a basestation 404 performs a process 500 of characterizing a radar pattern.Interference is detected by one or more network entity at S502. In oneembodiment, interference is detected by user equipment, and reported tothe network through an attached base station. In another embodiment,such as when interference is present in uplink frequencies, interferenceis detected at a base station.

In an embodiment, interference is detected through a communicationsmetric such as a Signal to Interference plus Noise Ratio (SINR). Inanother embodiment, the interference is detected by directly measuringsignal strength of radio frequencies. In such an embodiment, the signalstrength may be compared to a threshold value, and signals that exceedthe threshold value are classified as interference.

As discussed above with respect to FIG. 3B, a radar transmitter maytransmit at a regular interval so that network equipment in geographicalareas affected by the radar transmissions experience variations in radarsignal strength over time. If the signal strength of individual radarpulses 302 varies in a repeatable pattern, a dwell time may bedetermined at S504.

Determining a dwell time 504 may include comparing radar signal strengthor interference levels to a predetermined threshold value 320. Thethreshold value may differentiate an acceptable level of interferencefrom an unacceptable level of interference. For example, in anembodiment, the threshold interference value is −104 dBm for a 10 MHzchannel bandwidth. The dwell time 322 is the time for which radarinterference exceeds the threshold value 320. A separate dwell time maybe established for individual network equipment or geographical areas ina network.

In an embodiment, determining a dwell time S504 includes determiningtime characteristics of a dwell cycle, including a dwell start time, adwell end time and an interval between dwells. If radar oscillationsfollow a repeating pattern, determining a dwell time S504 may includedetermining all characteristics of the pattern so that a system canadapt to the dwell times 322. For example, the system may ceasetransmissions, adapt parameters or zero out transmissions during thedwell times. Processes that the system may perform during dwell timesare discussed in further detail below.

In an embodiment, a Pulse Repetition Interval (PRI) 304 is determined atS506, an on time 306 is determined at S508, and an off time 308 isdetermined at S510. When determining radar characteristics, variationsin the characteristics are also determined. For example, when theduration of pulse widths may vary over time, S508 includes determiningthis variance. Accordingly, information that is determined in processS500 can be used by a communications network 400 to efficiently coexistwith a radar system 402. The presence of radar transmissions may bedetermined by examining power, bandwidth, and pulse times.

The throughput of a link in a communications network is affected by thechannel quality of the link, measured in terms of the SINR, andtransmission parameters. In an embodiment, the transmission parametersinclude a Modulation and Forward Error Correction (FEC) coding scheme.For a given set of transmission parameters, the throughput reduces(eventually going to zero) as the channel quality of the link degrades.

Interference power degrades the SINR of the link. However, it may bepossible to maintain the communication link, e.g., to maintain non-zerothroughput, albeit with a lower throughput, if the transmissionparameters of the link are modified.

In contemporary communication systems, feedback mechanisms areimplemented between a receive node and a transmit node to adapt thetransmission parameters of the link according to the quality of the linkperceived at the receive node. For example, in LTE systems, userequipment sends back Channel Quality Information (CQI) reports to thetransmit node. A certain delay is associated with this feedbackmechanism. In addition, to reduce the overhead due to the feedbackmechanism, the feedback is typically based on average channel statisticsrather than instantaneous channel statistics. Due to these factors,transmission parameters of a link are maintained for a finite period oftime.

Radar systems may have pulse-widths on the order of a few tens ofmicroseconds and duty cycles on the order of hundreds of microseconds toseveral milliseconds. These time-scales over which interference from aradar system fluctuates are usually significantly smaller than thedurations over which the transmission parameters of a link are adaptedin response to channel quality feedback mechanisms. This leads tosub-optimal throughputs for the link since instantaneous variations inchannel quality due to radar interference fluctuations may not beexploited by the communication system.

In embodiments of this disclosure, information about the radar system isexploited to improve the throughput of the link. By performing process500 or through direct input, the start time, pulse width and PRI of theradar information is available at the communication system. From thisinformation, time durations during which the radar transmission ispresent, referred to here as radar on-time, can be determined by thecommunication system. If the transmissions of the radar system areperiodic for a significant period of time, it may also be possible forthe communication system to predict the time durations over which theradar transmission are present and interfere with the communicationsystem.

In a communication system, transmissions are typically scheduled infixed time intervals which are referred to here as Transmission TimeIntervals (TTIs). For example, in LTE systems, TTIs have a 1 msduration. FIG. 7 shows an example of cellular transmissions 700 whichinclude a plurality of sequential TTIs 702 transmitted over a timeperiod. In addition, FIG. 7 shows radar pulses 710 over the same timeperiod.

A process 600 for changing transmission parameters starts by identifyingaffected TTIs 702 at S602. Turning to FIG. 7, radar pulses 712 overlapwith TTIs 702 numbered 1, 3, and 5. Thus, the TTIs 702 numbered 1, 3,and 5 will experience interference from the radar transmissions, so theyare identified as affected TTIs at S602. In an embodiment in which anoscillating radar system is present, identifying TTIs may includeidentifying TTIs that correspond to dwell times 322.

In a communications system, a certain set of transmission parameters areused for a communication link in the absence of interference from aradar system. These parameters are continued to be used in the TTIs 702in which the radar on-time does not overlap with the TTI of thecommunication link. However, in the TTIs which overlap with radar pulses712, the transmission parameters are adjusted to maintain the link.

At S604, the SINR of affected TTIs may be determined. In an embodiment,the expected SINR of the communication link in the presence radarinterference is calculated based on location and transmit power levelinformation about the radar system so that transmission parameters canbe chosen based on expected performance. If the expected SINR cannot becalculated in advance, the SINR may be determined iteratively bymonitoring the link performance.

In process 600, transmission parameters are adjusted for affected TTIs.In an embodiment, the transmitter parameter adjustments that are madetrade the throughput of the link for improved received SINR perinformation bit. For example, a modulation rate may be lowered at S606so that there is more separation between the constellation points. Forexample, if a modulation rate that is used by a communications systemfor TTIs 712 that are not affected by radar transmissions is 64 QAM,S606 may include lowering the modulation rate to 16 QAM.

In an LTE system, about 30 different modulation and coding schemes (MCS)can be used for the transmission link between an eNodeB and userequipment. Higher order MCSs offer more throughput, but are moresusceptible to degradation from interference. Lower order MCSs tradethroughput of the link for larger SINR per information bit. Thus, in anLTE system, a lower order MCS scheme that maintains the link albeit at alower throughput is chosen for TTIs 702 in which interference isexpected at S606.

Similarly, at S608, a FEC is lowered for TTIs 702 which are affected byradar transmissions such as radar pulses 712. A lower FEC code rate hasa higher redundancy, and therefore is more robust to interference. Insome embodiments, both of the FEC and modulation rates are lowered foraffected TTIs 702. S606 and S608 may be performed a plurality of timesand regularly adjusted to optimize data transfer for a communicationssystem in the presence of radar interference.

FIG. 8 shows a process 800 of blanking the portions of transmittedsignals that are affected by radar. At S802, a communications signal isreceived by a network element such as a base station. More specifically,a communications signal that overlaps with a radar transmission isreceived at S802.

At S804, portions of the signal received at S802 that are affected by aradar transmission are identified. Identifying the affected portionsS804 may include determining interference that is present in the signal,or comparing times of radar pulses to signal reception times. Forexample, TTIs 702 may be divided into a number of portions, and thesystem may identify which portions of the TTIs overlap with radarpulses. In another embodiment, a value such as SINR may be measured foreach portion of the signal, and if the SINR is below a threshold for aparticular portion, S804 determines that particular portion is affected.

In LTE systems, each TTI 702 contains 14 OFDM symbols. Thus, anembodiment may divide the TTI into two halves and determine that aninterfering radar transmission was present for the first half of thesignal, thereby identifying the first 7 OFDM symbols as being affected.

At S806, the affected portion of the signal may be compared to apredetermined threshold value, and if the affected portion exceeds thepredetermined value, the affected portions of the signal are blanked atS810. For example, if S804 in an LTE system determines that sevensymbols are affected and the predetermined value is two, then the sevenaffected symbols are blanked out. Similarly, if S804 identifies one OFDMsymbol as being affected, then the signal is not blanked at S810. When aportion of a signal is blanked out at S810, that portion of the signalis not processed. Blanking out may include deleting the affected portionor instructing a processor to not process the affected portion.

At S808, the energy received from the radar transmission is compared toa threshold value. The threshold value may be set based on whether asystem can successfully process the signal in the presence of a radartransmission. In other words, the threshold value may be a value abovewhich there is a significant probability of processing errors.Particular thresholds may be calculated on the basis of simulationanalysis or performance measurements.

At S812, the received communications signal is processed. Blankedsubcarriers carrying data may be processed as though data with zeroenergy was received. Thus, if portions of the signal were affected byradar transmissions, then only unaffected portions of the signal areprocessed. However, if the affected portion of the signal was less thanthe predetermined threshold value of S808, then the entire signalincluding the affected portion may be processed. If subcarriers carryingreference signals used for channel estimation have been blanked, thenthe channel estimation algorithms may ignore these subcarriers whenmaking estimates of the channel gains.

It has been observed through experiments that zeroing out the receivedenergy improves the performance of the system as opposed to processingthe TTI 702 without any modifications when the received interferencepower from the radar transmission is high. A reason for the improvementis because high interference powers bias the statistics used in thedecoding, demodulation or equalizer processes at the receiver.

FIG. 9 shows a process 900 for blanking received energy that may beperformed without information from the radar system. Average energy in aTTI without interference is estimated at S902. The energy may beestimated based on key performance indicator (KPI) data, networkconditions, etc. Next, the TTI may be divided into subunits at S904. Forexample, in an LTE system, the TTI may be divided into as few as twosubunits of seven symbols, or as many as 14 subunits of a single symbol.

At S906, the energy of each subunit is determined, and at S908, a ratiobetween the energy of each subunit and the estimated average energy iscalculated for a subunit. At S910, a threshold value may be determinedon the basis of simulation analysis or performance measurements. In anembodiment, a threshold value may be determined once for a system.

At S912, the ratio between the received energy and the calculatedaverage of a subunit is compared to the threshold value from S910. Forexample, in an LTE system, in the 1 ms TTI, if the energy in an OFDMsymbol relative to the average energy of the block is greater than thethreshold value, the OFDM symbol is zeroed out. If the ratio exceeds thethreshold value, then the subunit is blanked at S914 as discussed withrespect to S810, and the next subunit is processed at S916.

A radar system is a pulsed system with a duty cycle that is typicallymuch less than 100%. Hence the off-time within a PRI 1004 of adwell-time is a significant portion of the PRI. If start time, pulsewidths and PRI information is available at the communication system,then the communication links may opportunistically scheduletransmissions during the off-times 1008. Such an embodiment is shown inFIG. 10.

As seen in FIG. 10, wireless transmissions such as TTI 1012 may betransmitted between radar pulses 1002, which is referred to here as anintra-dwell transmission since data transmissions in the communicationsnetwork take place during dwells. Intra-dwell transmissions are feasiblewhen the time of the TTI is less than the off-time 1008 between pulses1002.

In an embodiment in which radar off-times 1008 are significantly longerthan channel quality feedback times, intra-dwell scheduling can beimplemented even when no or limited information about the radar systemis available. The transmit node monitors the channel quality feedback.If the channel quality degrades, it can infer the presence of a radarpulse 1002. Correspondingly if the channel quality improves, the nodecan infer the off-time 1008 of the radar system.

FIG. 11 shows a process 1100 of scheduling transmissions in the presenceof radar. At S1102, aspects of a radar schedule are determined. Theradar schedule may be determined in accordance with process 500 ofcharacterizing a radar pattern as discussed above. Aspects of theschedule that may be determined at S1102 include a PRI 1004, an on time1006, an off time 1008, a receive time 1010 and a saturation time 1014.

Radars typically transmit at high power-levels and could saturate thereceive circuitry in communication nodes. If this happens and theoff-times 1008 are insufficient to alleviate the saturation condition ofthe receivers, the communication system may not be able to successfullytransmit in the off-times. Accordingly, determining the radar schedule1102 may include determining saturation time 1014.

At S1104, the process determines a time between pulses 1002. In oneembodiment, the time determined at S1104 may simply be the off time1008. In another embodiment, the time determined at S1104 may be the offtime 1008 minus the receive window 1010. In other embodiments, the timedetermined at S1104 may be the off time 1008 minus the saturation time1014, or the off time 1008 minus the greater of the receive window 1010and the saturation time 1014 (if the receive window 1010 overlaps withthe saturation time 1014) or the off time 1008 minus the receive window1010 and the saturation time 1014 (if the receive window 1010 does notoverlap with the saturation time 1014). A buffer of a predetermined timemay be subtracted from the off-time to account for other factors in acommunications system, such propagation time and schedule variance.

At S1106, a TTI 1012 is compared to the time determined at S1104. IfS1106 determines that the time between pulses is greater than the TTI1012, then intra-dwell scheduling may be performed at S1108. Intra-dwellscheduling may include scheduling one or more transmission 1012 in thetime between radar pulses 1002. As discussed above, the transmission1012 may be scheduled during an opportunity which may be defined by acombination of an off-time 1008 minus one or more of a receive window1010 and a saturation time 1014.

As discussed above with respect to FIG. 3B, a radar transmitter maychange direction over time to cover a larger geographic area than astationary radar. Such systems typically change direction in a regularrepeating pattern. A node will receive maximum energy levels from theradar when the radar is pointed directly at the node, and it willreceive decreasing levels of energy as the radar moves to point awayfrom the node.

FIG. 12 shows an embodiment of energy levels experienced by a node whichreceives energy from a radar transmission. As the radar points towardsthe node, energy levels reach a peak value at the pulse marked as 1202,and as the radar points further away from the node, energy levelsprogressively decrease. In an embodiment, radar energy levelsexperienced by the node decrease below a threshold value 1204 to definea transmission opportunity 1208.

Radar may not interfere with communication system during all pulses1202. The time during which pulses 1202 interfere with the communicationsystem above threshold 1204 are referred to here as interfering dwells1210. In an embodiment, threshold energy level 1204 is set at a pointabove which pulses 1202 are interfering pulses with substantial signaldegradation, and below which it is possible to transmit data 1206without substantial degradation. An example interference threshold valuefor a channel bandwidth of 10 MHz is −104 dBm. Therefore, S1110 mayinclude determining a transmission opportunity 1208 which is a timebetween interfering pulses 1202, or a time during which radar energy isexpected to be below threshold level 1204. The term transmissionopportunity may also be used with respect to a time between pulses 1202that is sufficient to schedule a TTI.

At S1112, the transmission opportunity 1208 is compared to atransmission time to transmit data 1206, which may be a TTI. If thetransmission opportunity 1208 is greater than the transmission time,then inter-dwell scheduling may be performed at S1114. Inter-dwellscheduling includes scheduling one or more transmissions during theopportunity 1208. If information about the dwell timing is available atthe communication system, then the communication links canopportunistically schedule transmissions during the opportunity 1208between interfering dwells 1202.

In an embodiment in which opportunities 1208 are longer than channelquality feedback times, inter-dwell scheduling can be implemented evenwhen no or limited information about the interfering dwell time 1210 isavailable. In such an embodiment, the transmit node monitors channelquality feedback, and if the channel quality degrades, the start of aninterfering dwell time 1210 can be determined Correspondingly, if thechannel quality improves, the end of the interfering dwell time 1210 canbe determined.

To prevent the links from transitioning to a disconnected or idle stateif there is inactivity in the link during the interfering dwell times1210, the connection timers associated with the communication systemscan be configured so that communications connections won't time outduring the interfering dwells. For example, in LTE systems, theconnection timer T310 in user equipment may be modified to maintainconnectivity in the presence of inactivity of the link from an eNodeB tothe user equipment.

What is claimed is:
 1. A method for scheduling wireless transmissions ina wireless communications network in the presence of pulsedinterference, the method comprising: receiving pulsed interferenceinformation; determining transmission opportunities based on the pulsedinterference information; and when a transmission time interval (TTI) isless than a time of an opportunity of the transmission opportunities,transmitting a wireless signal during the opportunity, whereindetermining transmission opportunities includes identifying off-timesbetween radar pulses and determining a buffer time between pulse times,and wherein the transmission opportunities are off times between pulsesminus the buffer.
 2. The method of claim 1, wherein the buffer time is areceive window in which a radar receives reflections.
 3. The method ofclaim 1, wherein the buffer time is a saturation time during which areceiver is saturated by a received pulse.
 4. The method of claim 1,wherein determining transmission opportunities further includesdetermining radar dwell times.
 5. The method of claim 4, whereindetermining radar dwell times includes: identifying a plurality ofconsecutive radar pulses; and comparing the plurality of consecutiveradar pulses to a threshold value, wherein the dwell time is a timeduring which a portion of the consecutive radar pulses exceeds thethreshold value.
 6. The method of claim 5, wherein additionaltransmission opportunities are identified as times between dwell times.7. A communications system, the system comprising: a receiver thatreceives pulsed interference information; a scheduling module thatdetermines transmission opportunities based on the pulsed interferenceinformation, and a transmitter that transmits a wireless signal duringan opportunity of the transmission opportunities when a transmissiontime interval (TTI) is less than a time of the opportunity, whereindetermining transmission opportunities includes identifying off-timesbetween radar pulses and determining a buffer time between pulse times,wherein the transmission opportunities are off times between pulsesminus the buffer.
 8. The communications system of claim 7, wherein thebuffer time is a receive window in which a radar receives reflections.9. The communications system of claim 7, wherein the buffer time is asaturation time during which a receiver is saturated by a receivedpulse.
 10. The communications system of claim 7, wherein the schedulingmodule determines radar dwell times.
 11. The communications system ofclaim 10, wherein the scheduling module determines radar dwell times byidentifying a plurality of consecutive radar pulses and comparing theplurality of consecutive radar pulses to a threshold value, and whereinthe dwell time is a time during which a portion of the consecutive radarpulses exceeds the threshold value.
 12. The communications system ofclaim 11, wherein additional the transmission opportunities are timespresent between dwell times.
 13. A method for scheduling wirelesstransmissions in a wireless communications network in the presence ofpulsed interference, the method comprising: receiving pulsedinterference information; determining transmission opportunities basedon the pulsed interference information; and when a transmission timeinterval (TTI) is less than a time of an opportunity of the transmissionopportunities, transmitting a wireless signal during the opportunity,wherein determining transmission opportunities includes: comparing a TTIto an off time between pulses; when the TTI is less than the timebetween pulses, identifying the transmission opportunities in timesbetween pulses; and when the TTI is greater than the time betweenpulses, determining whether a dwell time is present in a pulse pattern.14. A communications system, the system comprising: a receiver thatreceives pulsed interference information; a scheduling module thatdetermines transmission opportunities based on the pulsed interferenceinformation, and a transmitter that transmits a wireless signal duringan opportunity of the transmission opportunities when a transmissiontime interval (TTI) is less than a time of the opportunity, wherein thescheduling module determines transmission opportunities by: comparing aTTI to an off time between pulses; when the TTI is less than the timebetween pulses, identifying a transmission opportunity in the timebetween pulses; and when the TTI is greater than the time betweenpulses, determining whether a dwell time is present in a pulse pattern.